1. Field of the Invention
The present invention relates to a sheet forming machine which forms a slurry material into a sheet by use of a pair of coating members.
2. Description of Related Art
Generally, ceramic green sheets are made by casting. In the casting, a slurry material is continuously poured onto a traveling organic film as a sheet with a specified thickness, and the material on the organic film is dried and rolled up during the travel of the organic film. A sheet forming machine is used to carry out the casting.
Conventionally, a sheet forming machine is provided with a reverse roll coater as shown in FIG. 3, a knife coater as shown in FIG. 4 or the like so as to continuously pour a slurry material onto an organic film.
The reverse roll coater of FIG. 3 has a pair of coating rolls R.sub.1 and R.sub.2, and a back-up roll R.sub.3. The rolls R.sub.1, R.sub.2 and R.sub.3 are rotated in the same direction (clockwise direction in this example). The coating rolls R.sub.1 and R.sub.2 are arranged opposite each other with a specified space H in-between. While a slurry material S is poured through the space between the coating rolls R.sub.1 and R.sub.2, an organic film B is continuously fed by the back-up roll R.sub.3. Thereby, the material S poured through the space between the coating rolls R.sub.1 and R.sub.2 comes onto the organic film B, and the material S is made into a sheet F.
The knife coater of FIG. 4 has a coating knife N, a coating roll R.sub.2 and a back-up roll R.sub.3. The rolls R.sub.2 and R.sub.3 are rotated in the same direction (clockwise direction in this example). While a slurry material S is poured through the space H between the coating knife N and the coating roll R.sub.2, an organic film B is continuously fed by the back-up roll R.sub.3. Thereby, the material S poured through the space between the knife N and the roll R.sub.2 comes onto the organic film B, and the material S is made into a sheet F.
In the reverse roll coater of FIG. 3, the space H between the coating rolls R.sub.1 and R.sub.2 serves as an inlet of the material S and is an important factor which determines the thickness of the sheet F. Likewise, in the knife coater of FIG. 4, the space H between the coating knife N and the coating roll R.sub.2 is an important factor which determines the thickness of the sheet F. Therefore, in order to make a sheet F with a uniform thickness, the space H must be kept constant while the coating rolls R.sub.1 and R.sub.2 are rotating.
However, it is difficult to produce exactly cylindrical rolls R.sub.1 and R.sub.2 because of errors in processing, and the rolls R.sub.1 and R.sub.2 generally have out-of-roundness of about several micrometers to scores of micrometers. Also, even if an exact cylindrical roll can be produced, because of errors in providing a shaft to the cylindrical roll, eccentricity is generated. Accordingly, while the coating rolls R.sub.1 and R.sub.2 are rotating, the level of the circumferential point of each roll which faces the other roll changes periodically. Consequently, the space H between the rolls R.sub.1 and R.sub.2 in the reverse roll coater of FIG. 3 or the space H between the knife N and the roll R.sub.2 in the knife coater of FIG. 4 changes during the rotation of the coating rolls R.sub.1 and R.sub.2.
The change of the space H is described in connection with the reverse roll coater of FIG. 3. As shown in FIG. 2a, the coating roll R.sub.1 is rotated at a relatively low speed, and the level of the circumferential point facing the roll R.sub.2 changes periodically as indicated by a curve C.sub.1. The coating roll R.sub.2 is rotated at a higher speed, and the level of the circumferential point facing the roll R.sub.1 changes periodically as indicated by a curve C.sub.2. The change of the space H between the rolls R.sub.1 and R.sub.2 is indicated as the change of the distance between the curves C.sub.1 and C.sub.2. The space between the rolls R.sub.1 and R.sub.2 is H.sub.1 at a moment and is H.sub.2 (.noteq.H.sub.1) at another moment.
In accordance with the change of the space H between the rolls R.sub.1 and R.sub.2, the thickness of the material S poured through the space H changes. Thus, it is difficult to form a sheet F with a uniform thickness.